Cut the Rope - awesome logic problems
Angry Birds - who'd have thought Balista would be popular again?
Pac-man - mega hard on Accelerometer mode
Absolute Radio - beats Vibe FM
Tetris - doesn't need an explanation, really
iDragPaper - try it
iRepton - spent countless hours playing this as a child on the BBC, now I can do it while having a poo at work
Sky+ - for when you can't reach the remote

Still 15GB left for more